The instant invention relates to mechanical packing materials for controlling leakage around shafts and other cylindrical or curved surfaces, and more particularly to a packing material which is formed using previously knit interlace or filler yarns which will naturally expand and compress in a longitudinal direction when the packing is wrapped around a shaft, and thus reduce keystoning of the packing material.
In general, mechanical packings are usually manufactured in relatively long coils of packing material of square or rectangular cross-section from which many individual lengths can be cut to form packing rings. Conventionally, a packing is formed by cutting several lengths from the coil of material, each length then being formed into a ring with abutting ends and installed around a shaft or around another cylindrical object. One longstanding drawback of existing packing materials is a deformation, i.e. bunching or "keystoning"of the packing material when it is bent around corners, or around a shaft. "Keystoning" is generally defined as a bunching of material along the inner radius of a bend when the material is wrapped around a corner or curved surface. The extra material on the inner radius of the corner bunches up and creates wrinkles at the corner which in turn create pressure points, and reduce facing contact with the surrounding structures. Although there have been many attempts to reduce keystoning by changing the initial shape of square or rectangular packings to a trapezoidal formation so that keystoning is reduced when wrapped, the problem still exists and still reduces the effective life of the existing packing materials.
The instant invention provides an improved packing material of square, rectangular or other geometric construction, which reduces keystoning of the packing material. In particular, the improved packing material comprises an elongate cord-like construction having an inner core and an outer sheath that is constructed from a plurality of base yarns and a plurality of interlace or filler yarns. The inner core can comprise a variety of different types and constructions of varying materials including bundles of straight yarns, knit bundles of yarns, knit or braided cords, etc. The outer sheath is knit or braided around the core in a tubular configuration wherein the base yarns comprise yarn compositions which are typically used to form packing materials, such as Teflon, glass, Kevlar, graphite, etc. As will be further described herein, it is known in both braiding and knitting techniques to insert "filler" yarns within the construction to achieve certain desired characteristics to the cords. In the braiding art, such yarns are typically referred to as "filler" yarns, and in the warp-knitting art, the yarns are typically referred to as "interlace or inlaid" yarns. To simplify further discussion of the sheath construction, it is to be understood by the reader that the term "filler" yarn will be used interchangeably with respect to both knitting and braiding techniques, and that the term "filler" is intended to cover either an interlace yarn in knitting or a filler yarn in braiding.
Continuing on with the sheath structure, the filler yarns are selectively positioned at predetermined locations within the sheath structure so as to give the sheath a predetermined geometric configuration when formed. In the preferred construction, four filler yarns are arranged in a square configuration to give the packing material a generally square configuration. The key feature of the present invention is that the filler yarns comprise natural yarn materials or yarn constructions which are naturally extendible and compressible in a longitudinal direction. In this regard, the filler yarns preferably comprise a chain stitched knit yarn construction which is naturally extendible and compressible in a longitudinal direction. The previously knit filler yarns are introduced into the knit or braided sheath structure in partially extended condition, i.e. under back tension, and will thereafter be at least partially compressible and extendible in a longitudinal direction in the finished packing material. In this regard, the sheath is at least partially longitudinally compressible along an inner radius when the packing material is bent around a curved surface. The resulting effect of the use of the previously knit filler yarns is a noticeable reduction of "keystoning" of the packing material along the inner radius. Although it is preferred to utilize the previously knit filler yarns on all corners of the packing, it is to be understood that the filler yarns are really only required on the innermost corners because keystoning is only a significant problem on the inner radius of a packing ring.
Accordingly, among the objects of the instant invention are: the provision of a packing construction which reduces "keystoning" of the packing when wrapped around corners or curved surfaces; the provision of a packing material having a sheath including filler yarns which are naturally compressible and extendible in a longitudinal direction; the provision of a packing material having a circular warp-knit or braided sheath including filler yarns which are naturally compressible and extendible in a longitudinal direction; and the provision of such a packing material wherein the compressible yarns preferably comprise knit yarn constructions.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.